Signing in and signing out
Apple, Google or an emailed link. No password, ever.
Zukan signs you in with Apple, Google or an emailed link. There is no password: with email you receive a fresh link each time, and opening it signs you in on the device that asked for it.
Settings → Account shows the address you are signed in with and how. Sign out leaves your collection saved to your account; sign back in on any device to find it again.
Signing in needs the internet. Everything else in the app works offline.